(a) Direct system-

(i) Ideal to small installations

(ii) Hot water from a boiler can be tapped directly for use.

(iii) 55-65°C temperature.


(iv) If the water has a pH lower than 7, its corrosive effects increases with rise in temperature and corrodes the boiler. This also makes the water to appear.

(v) When drawn off directly, the cold water entering the boiler near the bottom comes in contact with the boiler furnace, which is at a high temperature. T he contact generates a thermal imbalance, which may strain the body of the boiler and may develop cracks on the surface sheet of the furnace.

(vi) Boiler outlet nozzles must be large enough to match the rate of flow generated by the re circulating water in the distribution system.

(b) Indirect System:


(i) Hot water can also be generated indirectly by using the hot water or steam as a heating medium.

(ii) Hot water or steam generated in a boiler is circulated through a set of coils or tube bundle, in a primary circuit, which acts as the heat source.

Forced Hot Water Circulation –

(1) Generate pressure within the system to force the circulation by means of a pump. This enables the reduction of pipe sizes and allows hot water to circulate in all parts of the system for quick draw off.


(ii) The circulation pump must be designed to overcome frictional losses within the circuit for a flow rate calculated for the heat losses.

(iii) The circulating pumps should never be used as booster pumps for in­creasing the pressure in the hot water lines. The source of pressure in the hot water system should be the same as that for the cold water supply so as to have nearly equal pressure at the point of supply.

Limitations of Hot-Water Circulation:

When water is drawn off from a circulating system, the circulation tempo­rarily ceases and open taps draw water from the flow and return lines. Return pipes should therefore be designed to allow a partial draw off of hot water.


This method not only reduces the pipe diameter of flow line but also utilizes the return line more efficiently. Circulation in this system occurs only in a steady state and in sections of the system where no heavy drain off water is being made.